1. Field of the Invention
The present invention relates generally to a fluid-filled elastic mount used as a vibration damper such as an engine mount for a motor vehicle, and more particularly to such a fluid-filled elastic mount having a fluid chamber partially defined by an oscillating plate which is actuated by an electromagnetic drive device so as to electrically control the damping characteristics of the mount. The present invention is also concerned with a method of manufacturing the fluid-filled elastic mount having an electromagnetic drive device capable of producing a large drive force to actuate the oscillating plate.
2. Discussion of the Prior Art
As a vibration damper such as an engine mount for a motor vehicle, for flexibly connecting two members in a vibration system or mounting one of the two members on the other member in a vibration damping fashion, there is known an elastic mount interposed between the two members of the vibration system. The elastic mount has an elastic body interposed between and elastically connecting a first and a second support, which are spaced apart from each other in a load receiving direction in which a vibrational load is applied to the mount. The first and second supports are secured to the respective ends of the elastic body which are opposed to each other in the load receiving direction, and are fixed to one and the other of the two members of the vibration system. This type of elastic mount may be used as a suspension bushing for the motor vehicle, as well as the engine mount indicated above.
Recently, there have been proposed various types of fluid-filled elastic mounts adapted to exhibit sophisticated damping characteristics, wherein the elastic body which elastically connects the first and second supports partially defines a fluid chamber filled with a non-compressible fluid. Some of these fluid-filled elastic mounts are adapted to electrically control the fluid pressure within the fluid chamber, depending upon the type of the input vibrations received, so that the specific vibrations can be suitably damped or isolated.
Examples of such electrically controllable fluid-filled elastic mount are disclosed in JP-A-60-8540, JP-A-59-1828 and JP-A-59-1829, wherein the fluid chamber is partially defined by an oscillating plate which consists of a magnetic body or a permanent magnet. The oscillating plate is actuated by a solenoid coil, so as to produce pressure pulsation within the fluid chamber to thereby control the fluid pressure within the chamber. Another type of electrically controllable fluid-filled elastic mount is disclosed in Publication No. 3-73741 of Japanese Utility Model Application, wherein the fluid chamber is partially defined by an oscillating plate fixed to one of an annular permanent magnet and an annular coil which is disposed radially inward or outward of and concentrically with the permanent magnet. The oscillating plate is oscillated by an electromagnetic force produced upon energization of the coil, so as to suitably control the fluid pressure within the fluid chamber.
In the known electrically controllable fluid-filled elastic mount constructed as described above, the operation of the oscillating plate cannot be suitably regulated so as to enable the elastic mount to exhibit satisfactory damping characteristics, since it is difficult to obtain a sufficient drive force to effectively and stably actuate the oscillating plate.
Described more specifically, the above fluid-filled elastic mount suffers from insufficiency of the magnetic flux density in the magnetic field in which the oscillating plate or solenoid coil is placed, because the magnetic path or circuit formed by the solenoid or permanent magnet is open. In particular, the open magnetic circuit leads to insufficiency of the drive force to actuate the oscillating plate so as to effectively regulate the fluid pressure within the fluid chamber, when the elastic mount receives a vibrational load of medium to low frequencies having a relatively large amplitude. Further, the electromagnetic drive device including the solenoid coil and permanent magnet tends to be large-sized, thereby resulting in the elastic mount having a relatively large size.
Moreover, when the oscillating plate is actuated or displaced in the oscillating manner, the open magnetic circuit formed by the solenoid or permanent magnet inevitably causes a large variation in the magnetic flux density in the field in which the oscillating plate or coil is placed. As a result, the drive force which acts on the oscillating plate tends to be unstable, making it difficult to effectively control the oscillating plate, whereby the waveform of the pulsation induced within the fluid chamber is distorted, causing a fluid pressure control distortion of the fluid chamber. Thus, the known electrically controllable fluid-filled elastic mount is not satisfactory in terms of its damping characteristics.
Although the oscillating plate can be suitably controlled to regulate the fluid pressure within the fluid chamber so as to exhibit an intended damping effect with respect to the vibrations in a specific frequency band, the distortion of the pulsation waveform of the fluid would result in adversely amplifying the vibrations in another frequency band. Thus, the known fluid-filled elastic mount as a whole is incapable of exhibiting the intended damping characteristics over a wide range of frequency of the input vibrations.